This invention is concerned with a process for the preparation of polyimide derivatives in water and with polymers, particularly high-temperature-resistant polymers, made therefrom.
Polyimides are synthetic organic resins characterised by repeating imide linkages in the polymer chain which may or may not be end-capped with polymerisable or inert (i.e. non-polymerisable) chemical groups. They are available in both linear and cross-linked forms and are noted for their excellent thermal, mechanical and dielectric properties, such as high glass transition temperature, high thermal decomposition temperature, and high mechanical strength. In addition to their use as matrices for fibre reinforced composites, they may be used as precured films and fibres, curable enamels, laminating resins, adhesives, and for forming molded articles.
The standard method for the synthesis of polyimides involves reacting aromatic diamines with purified dianhydrides and optionally functionalised monoanhydrides in dry and highly purified polar solvents at room temperature. The resulting amic acids are then either heated to above 180.degree. C. or chemically cyclodehydrated to complete the cyclisation and form the imide ring structure.
The solvents chosen for these reactions have typically been those which would dissolve at least one of the reactants and preferably both the anhydrides and the diamines. It is also typically preferred that the solvent should maintain the resulting poly(amic acid) or poly(amic ester) in solution. Since the amines and anhydrides used for these applications are generally highly aromatic and often have limited solubility, the solvents that are chosen usually include N,N-dimethylformamide, N,N-dimethylacetamide, m-cresol, dimethylsulfoxide, N-methyl pyrrolidine, tetramethylurea, and the like. These solvents can be used alone or in combination with other solvents such as benzene, benzonitrile, dioxane, xylene, toluene and cyclohexane (see, for example, U.S. Pat. No. 5,138,028).
However, the use of these organic solvents introduces environmental and safety issues related to the handling and disposal of the solvents, and also issues of solvent cost. Another drawback of this approach is that the high boiling points of many of these solvents, coupled with their tendency to form complexes with the polymer (see, for example, D. Frank-Susich, D. H. Laananen and D. Ruffner, "Cure cycle simulation for thermoset composites" in Composites Manufacturing, Vol. 4, No 3, 1993, pp. 139-146), makes removal during processing difficult. This can lead to defects through the formation of voids. The presence of residual solvent may possibly be associated with long-term thermal instability via chemical interactions with the imide functionality. In addition, amide solvents also often contain mono functional amine impurities. These monofunctional amine impurities can compete with the monomeric diamines throughout the polymerisation cycle leading to chain termination. Such reactions can upset the monomer stoichiometry and lead to lower or variable molecular weights and unreactive chain ends.
It would therefore be advantageous for several reasons if these imidisation reactions could be performed in water.
However, since the reactions to form amic acids and imides are reversible, and water is a product of the reaction, it appears that the presence of water in this process would be highly undesirable. It is generally accepted that for the production of high molecular weight poly(amic acids) a major disadvantage of the dianhydride/diamine route is the extreme moisture sensitivity of the initial stage since trace amounts of water may rapidly hydrolyse the dianhydride monomer and poly(amic acid), preventing the attainment of high molecular weight poly(amic acid). Also since the aromatic reactants are generally at the most only partially soluble in water, and are often totally insoluble, it would seem to be unlikely that any reaction between the anhydride and amine components should occur readily or to completion.
Despite this, it has now been surprisingly found that water can be used as the reaction medium in a process for forming oligomeric polyimides.
There are examples of aromatic poly(amic acids) being synthesised in water or mixed water/organic solvents (see, for example, J. Ookawa et al; Japanese Patent 95-139436, 1995; H. Nomura and T. Goto, Japanese Patent 94-144623, 1994; H. Nishizawa et al, Japanese Patent 87-4296, 1992). However, in all cases where water alone is used as a solvent, a less reactive amine or nitrogenous base, such as a secondary or tertiary amine or pyridine, has been added to stabilise the resulting amic acid and to aid its dissolution, notionally through the formation of a salt at the free carboxylic acid functionality. A recent variant on this procedure (J. V. Facinelli et al, Macromolecules, 1996, Vol 29, pp. 7342-7350) was to generate the poly(amic acid) salts from preferred poly(amic acids) in an organic solvent such as NMP or THF. The solvent mixture, including water, was in all cases removed from these poly(amic acid) salt solutions, by drying, before imidisation.
These methods suffer from a disadvantage in that, when water is used as a solvent alone, there is a byproduct formed in the imidisation reaction--the basic counterion which was used to produce the amic acid salt--which may not be completely removed. If mixed aqueous/organic solvents are used, there will also still be the disadvantage that organic solvents will be introduced which may also be difficult to completely remove. Also, the fact that the procedures require drying of the intermediate amic acid salts before the imidisation process begins is a further disadvantage in that a separate stage of processing is required.
Two procedures exist in which an imide is formed in an aqueous solvent. In one process, diaminobisimides are prepared in water upon heating to between 120.degree. C. and 200.degree. C. (J. H. Hodgkin et al in Australian Patent 647,537). However, this example is only of the formation of a monomeric diaminobisimide with only two imide formation steps, and the insolubility of the resulting product is an advantage in this case as it is required that further reaction be prevented. Further, for the same reason it was required that there be either good conjugation between the nitrogen atoms of the aromatic diamine, or that steric or other restrictions capable of moderating reactivity of the unreacted amine moiety were present, to limit the number of imide formation steps to that of the monomer required. Likewise, in the second example (U.S. Pat. No. 5,264,588) the objective of the synthesis was the formation of a monoimide, chloro-N-phenylphthalimide. The procedure required that a chlorophthalic acid compound and aniline were heated in water to between 95.degree. C. and 105.degree. C. to give the monoimide product which precipitated out on cooling.
It is therefore an unexpected result that polyimides, as opposed to monomeric mono- or diimides of low molecular weight, may also be formed in water. This is particularly true given that the insolubility of the reagents in water mean that many complex imide formation reactions must occur in products which are insoluble and in an environment which should in theory promote the reverse process.